Ghaba Salt Basin Province and Fahud Salt Basin Province, Oman—

Geological Overview and Total Petroleum Systems

By Richard M. Pollastro

U.S. Geological Survey Bulletin 2167

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

Bruce Babbitt, Secretary

U.S. Geological Survey

Charles G. Groat, Director

This report is only available on-line at:

http://greenwood.cr.usgs.gov/pub/bulletins/b2167/b2167.html

Any use of trade, product or firm names is for

descriptive purposes only and does not imply

endorsement by the U.S. Government.

Published in the Central Region, Denver, Colorado

Manuscript approved for publication June 28, 1999

Graphics by the author

Photocomposition by the author

Edited by Lorna Carter
iii

CONTENTS

Foreword 1

Acknowledgments 2

Abstract 2

Introduction 3

Province Geology and Petroleum Occurrence 5

Province Boundaries 5

Structural Setting 5

Stratigraphy 9

Petroleum System Overview 11

Hydrocarbon Exploration and Production History 14

Total Petroleum Systems and Assessment Units 18

Ghaba Salt Basin Province (2014) 18

North Oman Huqf/“Q”–Haushi(!) Total Petroleum System (201401) 18

Reservoirs, Seals, and Trap Styles 20

Source Rock Character and Geochemistry 20

Burial History, Generation, and Migration 21

Ghaba-Makarem Combined Structural Assessment Unit (20140101) 24

Fahud Salt Basin Province (2016) 25

North Oman Huqf–Shu’aiba(!) Total Petroleum System (201601) 25

Reservoirs, Seals, and Trap Styles 26

Source Rock Character 28

Burial History, Generation, and Migration 28

Fahud-Huqf Combined Structural Assessment Unit (20160101) 28

Middle Cretaceous Natih(!) Total Petroleum System (201602) 31

Source Rock Lithology and Geochemistry 31

Reservoirs, Seals, and Trap Styles 31

Burial History, Generation, and Migration 33

Natih-Fiqa Structural/Stratigraphic Assessment Unit (20160201) 33

Summary 36

References Cited 37
i v

FIGURES

1. Map showing U.S. Geological Survey Province boundaries, names, and numbers,

location of burial histories, and oil and gas field centerpoints 4

2. Map showing salt basins, structural elements, and major oil and gas fields of Oman 6

3. Generalized northwest-southeast cross section across study regions of Oman 7

4. Generalized east-west cross section across central Ghaba Salt Basin 8

5. Stratigraphic section of Oman showing source rocks and producing

reservoirs for Ghaba and Fahud Salt Basins 10

6. Map showing geographic distribution of oil types found in Oman and approximate

“Q” oil migration fairway 12

7. Diagram showing various populations for Oman oil types 13

 8–10. Charts of petroleum system events for:

8. North Oman Huqf/“Q”–Haushi(!) Total Petroleum System 15

9. North Oman Huqf–Shu’aiba(!) Total Petroleum System 16

10. Middle Cretaceous Natih(!) Total Petroleum System 17

11. Map showing Ghaba Salt Basin Province 19

12. Burial-history diagram, vitrinite reflectance equivalent, and stages of hydrocarbon

generation for Huqf source rocks, Ghaba Salt Basin Province 22

13. Burial-history diagram for deep-gas reservoirs, Barik Sandstone Member, Ghaba

Salt Basin 23

14. Map showing North Oman Huqf–Shu’aiba(!) Total Petroleum System and related data 27

15. Burial-history diagram, vitrinite reflectance equivalent, and stages of hydrocarbon

generation for Huqf source rocks, Fahud Salt Basin Province 29

16. Burial-history diagram for deep-gas reservoirs, Barik Sandstone Member, Fahud

Salt Basin 30

17. Map showing North Oman middle Cretaceous Natih(!) Total Petroleum

System and related data 32

18. Burial-history diagram, burial temperatures, and vitrinite reflectance

equivalent for Natih Formation source rocks, Fahud Salt Basin 34

19. Burial-history diagram and major diagenetic events of Wasia Group near Anaima-1 well,

Fahud Salt Basin 35

TABLES

1. Common characteristics of source rocks and oils of Ghaba and Fahud Salt Basin Provinces,

 north-central Oman 40

2. Allocation of known oil and gas fields to Total Petroleum Systems and Assessment Units of the

Ghaba and Fahud Salt Basin Provinces, Oman 41
1

Ghaba Salt Basin Province and Fahud Salt Basin Province,

Oman—Geological Overview and Total Petroleum Systems

By Richard M. Pollastro

Foreword

This report is a product of the World

Energy Project of the U.S. Geological Survey, in

which the world has been divided into 8

regions and 937 geologic provinces for purposes

of assessment of global oil and gas resources

(Klett and others, 1997). These provinces have

been ranked according to the discovered

petroleum volumes within each; 76 "priority"

provinces (exclusive of the U.S. and chosen for

their high ranking) and 26 "boutique" provinces

(exclusive of the U.S. and chosen for various

reasons) were selected for appraisal of oil and

gas resources. Assessment results will be

released in a later report. More specific digital

compilations of the geology and province

boundaries of the Arabian Peninsula are also

available on CD-ROM (Pollastro and others,

1998).

The Total Petroleum System constitutes the

basic geologic unit to be assessed for

undiscovered oil and gas resources. A Total

Petroleum System includes the essential

elements and processes, as well as all

genetically related hydrocarbons, that occur in

petroleum shows, seeps, and accumulations

(discovered and undiscovered) whose

provenance is a pod or closely related pods of

mature source rock (modified from Magoon and

Dow, 1994). The minimum petroleum system is

that portion of the Total Petroleum System for

which the presence of essential elements and

processes has been proved.

An assessment unit is a mappable volume of

rock within the Total Petroleum System that

encompasses petroleum fields (discovered and

undiscovered) which share similar general

geologic traits and socio-economic factors. The

fields in an assessment unit should constitute a

population that is sufficiently homogeneous in

terms of geology, exploration strategy, and risk

so that the chosen methodology of resource

assessment is applicable. A Total Petroleum

System might equate to a single assessment

unit. If necessary, a Total Petroleum System can

be subdivided into two or more assessment units

in order that each assessment unit is

sufficiently homogeneous. Each assessment unit

can incorporate several exploration plays based

on different reservoir formations, trap types,

exploration strategies, and discovery histories.

Assessment units are considered established if

they contain more than 13 fields, frontier if

they contain 1–13 fields, and hypothetical if

they contain no fields.

A numeric code identifies each region,

province, Total Petroleum System, and assessment unit; these codes are uniform throughout

the project and will identify the same item in

any of the publications. The code is as follows:

Example

Region, single digit 3

Province, three digits to the right of region

code 3162

Total Petroleum System, three digits to the

right of province code 3162050

Assessment unit, one digit to the right of

petroleum system code 31620504

The codes for the regions and provinces were

established, listed, and mapped in Klett and

others, 1997.

The purpose of describing the Total

Petroleum Systems of this area is to aid in

assessing the quantities of oil, gas, and natural

gas liquids that have the potential to be added

to reserves within the next 30 years. These

volumes either reside in undiscovered fields

whose sizes exceed the stated minimum-fieldsize cutoff value for the assessment unit or occur

as reserve growth of fields already discovered.

"Reserves" quoted in this report are from

Petroconsultants, Inc., 1996 Petroleum

Exploration and Production database and other

area reports from Petroconsultants, Inc., unless

otherwise stated.

Figure(s) in this report that show

boundaries of the Total Petroleum System(s),
2

assessment units, and pods of active source rock

were originally compiled using geographic

information system (GIS) software and later

drafted into the present illustrations.

Political boundaries and cartographic

representations were taken, with permission,

from ESRI's 1992 ArcWorld 1:3 million digital

coverage, have no political significance, and

are displayed for general reference only. Oil

and gas field centerpoints, shown in drafted

figure(s), are reproduced with permission from

Petroconsultants, Inc., 1996 Worldwide Oil and

Gas Field database.

Also included as figures are events charts

that show the time of deposition of essential

rock units; the time processes, such as trap

formation, necessary to the accumulation of

hydrocarbons took place; the critical moment in

the petroleum system; and the preservation

time, if any. The format is that proposed by

Magoon and Dow (1994).

Acknowledgments

This manuscript has benefited considerably

from the scientific contributions and assistance

from several individuals. In particular, I

especially thank Joachim Amthor, Jos Terken,

Graham Tiley, and Mike Naylor of Petroleum

Development Oman, and Wallace Pierce of

Amoco Production for contributing information

and data, and participating in discussions on

the petroleum systems of Oman. I would also

like to thank Tim Klett, Tom Ahlbrandt, Chris

Schenk, and Jim Schmoker for their suggestions

and discussions during the preparation of this

report. The manuscript has improved greatly

from critical reviews by Chris Schenk and Tom

Ahlbrandt and thorough editing by Lorna

Carter. Thanks also to Felix Persits for his

assistance in preparing the digital maps.

Abstract

Three Total Petroleum Systems each

consisting of one assessment unit have been

identified in the Ghaba and Fahud Salt Basin

Provinces of north-central Oman. One Total

Petroleum System and corresponding assessment

unit, the North Oman Huqf/“Q”–Haushi(!)

Total Petroleum System (201401) and GhabaMakarem Combined Structural Assessment Unit

(20140101), were identified for the Ghaba Salt

Basin Province (2014). In the Fahud Salt Basin

Province, however, two overlapping Total

Petroleum Systems (TPS) were recognized: (1)

the North Oman Huqf–Shu’aiba(!) TPS

(201601); Fahud-Huqf Combined Structural

Assessment Unit (20160101), and (2) the middle

Cretaceous Natih(!) TPS (201602); Natih-Fiqa

Structural/Stratigraphic Assessment Unit

(20160201). The boundary for each Total

Petroleum System also defines the boundary of

the corresponding assessment unit and includes

all trap styles and hydrocarbon-producing

reservoirs within the petroleum system.

In both the Ghaba and Fahud Salt Basin

Provinces, hydrocarbons were generated from

several deeply buried source rocks within the

Infracambrian Huqf Supergroup. One general

“North Oman Huqf” type oil is dominant in the

Fahud Salt Basin. Oils in the Ghaba Salt

Basin are linked to at least two distinct Huqf

source-rock units based on oil geochemistry: a

general North Oman Huqf-type oil source and a

more dominant “questionable unidentified

source” or “Q”-type Huqf oil source. These two

Huqf-sourced oils are commonly found as

admixtures in reservoirs throughout northcentral Oman.

Hydrocarbons generated from Huqf sources

are produced from a variety of reservoir types

and ages ranging from Precambrian to

Cretaceous in both the Ghaba and Fahud Salt

Basin Provinces. Clastic reservoirs of the

Gharif and Al Khlata Formations, Haushi

Group (middle Carboniferous to Lower

Permian), dominate oil production in the

Ghaba Salt Basin Province and form the basis

for the Huqf/“Q”–Haushi(!) TPS. In contrast,

the Lower Cretaceous Shu’aiba and middle

Cretaceous Natih limestones account for most of

the production in the Fahud Salt Basin with

about 50 percent of the basin’s production from

porous, fractured Shu’aiba limestones in Yibal

field, thus the name North Oman Huqf–

Shu’aiba(!) TPS. Deep gas is produced mainly

from Middle Cambrian to Lower Ordovician

clastic reservoirs of the Haima Supergroup.

Traps in nearly all hydrocarbon accumulations

of these petroleum systems are mainly

structural and were formed by one or more
3

mechanisms. These trap-forming mechanisms

were mainly periodic halokinesis of the thick

Cambrian Ara Salt and consequent folding and

faulting from basin loading, rifting, or other

major tectonic events, particularly those events

forming the Oman Mountains and associated

foreland-basin system during the Late

Cretaceous and late Tertiary.

Many of the future new-field targets will

likely be low-relief, subtle structures, as many

of the large structures have been drilled.

Oman’s recent interest and commitments to

liquid natural gas export make deep gas a

primary objective in the two North Oman Huqf

petroleum systems. New-field exploration of

deep gas and exploring deeper targets for gas in

existing fields will likely identify a

significant gas resource in the next 30 years.

Moreover, salt-diapir flank traps in these two

North Oman Huqf petroleum systems and salt

basin provinces have gone essentially untested

and will likely be targeted in the near future.

The middle Cretaceous Natih(!) TPS is a

small efficient system of the Fahud Salt Basin.

Natih source rocks are only mature in the Late

Cretaceous/Tertiary foredeep and production is

primarily from Natih reservoirs; minor

production from the Shu’aiba limestone is

documented along fault-dip structures. Most

traps are structural and are related to

development of the foreland basin and

formation of the Oman Mountains. Future

targets of the Natih TPS will be less obvious

than those of the Fahud and Natih fields and

likely include smaller structural closures along

the northern flank of the foreland bulge and

traps above salt domes with late Tertiary

movement. Frontier exploration is predicted to

be mainly in stratigraphic traps within Natih

buildups and in unproven turbidite and other

marine clastics of the Fiqa Formation.

Introduction

The U.S. Geological Survey, World Energy

Project, has divided the world into 8 regions

and 937 geologic provinces for purposes of

assessment of global oil and gas resources.

These provinces have been ranked according to

the discovered petroleum volumes within each;

76 “priority” provinces (high ranking and

exclusive of the U.S.) and 26 “boutique”

provinces (chosen and exclusive of the U.S.)

were designated for appraisal of oil and gas

resources (Klett and others, 1997). The Ghaba

and Fahud Salt Basin Provinces of Oman

(USGS World Energy Project provinces 2014 and

2016, respectively; see Pollastro and others,

1998) are two of the 76 “priority” provinces;

the Ghaba Salt Basin Province ranked number

69 and the Fahud Salt Basin Province ranked

number 59. Total recoverable reserves are

estimated at 5.0 billion barrels of oil

equivalent (BBOE) for the Ghaba Salt Basin

Province and 6.3 BBOE for the Fahud Salt

Basin Province (Klett and others, 1997). The

location and boundaries of these provinces are

shown in figure 1. The largest field in the

Ghaba Salt Basin is Saih Rawl (fig. 2). Yibal

field (fig. 2) is the largest field of the Fahud

Salt Basin, and in Oman in general; early

conservative estimates of in place reserves at

Yibal have been reported at about 3.0 billion

barrels of stocktank oil (Litsey and others,

1983). Moreover, Yibal accounts for about 25

percent of Oman’s developed reserves, currently

producing about 217,000 barrels of oil per day

(BOPD) (Knott, 1998).

The Total Petroleum System concept is the

basis for this assessment. A Total Petroleum

System includes the essential elements and

processes, as well as genetically related

hydrocarbons that occur in petroleum shows,

seeps, and accumulations (discovered and

undiscovered), whose provenance is a pod or

pods of mature source rock (concept modified

from Magoon and Dow, 1994). The minimum

petroleum system is that portion of the system

for which the presence of essential elements

and processes has been proved. The assessment

unit is a mappable volume of rock within the

Total Petroleum System, sufficiently

homogeneous in terms of geology, exploration

strategy, and risk characteristics to constitute a

single population with respect to criteria used

for the chosen methodology of resource

assessment. The assessment unit, however, may

be limited to the data and information

available for assessment of the resource(s), thus

limiting the degree of “homogeneity.”

Assessment units are considered established if

they contain more than 13 fields, frontier if
Oman

Arabian Sea

Gulf of Oman

South Oman

Salt Basin

(2011)

Ghaba Salt Basin

(2014)

Fahud Salt Basin

(2016)

Central Oman Platform

(2015)

Huqf- Haushi Uplift

(2013)

Maradi Fault Zone

Rub' al Khali Basin

(2019)

East Flank Oman Sub-basin

(2012)

Oman Mountains

(2017)

Gulf of Oman Basin

(2018)

55° E. 56° E. 57° E. 58° E. 59° E.

21°

23°

24°

22°

20°

19°

Saudi

Arabia

U.A.E.

Oil or gas field centerpoint

USGS Province boundary (2014) USGS Province number

Cross section

Burial history

and location

Figure 4

Figure 3

EXPLANATION

Fahud-Natih

faults

0 100 km

B

A

C

D

E

F

A

Figure 1. Map showing U.S. Geological Survey (USGS) Province boundaries, province names

and numbers, location of burial histories, and oil and gas field centerpoints (Petroconsultants,

1996) in Central and North Oman. Scale: 1:3,600,000. [Burial history location for: A, figure 12

near Saih Rawl field; B, figure 13 near Barik field; C, figure 15 near Yibal field; D, figure 15 near

Jaleel field; E, figure 18 in Omani foredeep; F, figure 19 near Anaima-1 well.]

4
5

they contain 1–13 fields, and hypothetical if

they contain no fields.

A unique eight-digit numeric code

identifies each assessment unit with respect to

the region, province, and Total Petroleum

System. The first digit is the region number, the

next three digits the province number, the next

two digits the Total Petroleum System number,

and the final two digits are the assessment

number. The codes for the regions and provinces

were established, listed in Klett and others

(1997); provinces for the Arabian Peninsula are

listed and shown in greater detail in Pollastro

and others (1998).

Oil and gas production in Oman has grown

considerably in the past 36 years to a present

level of about 900,000 BOPD (U.S. Energy

Information Administration, 1998; Petroleum

Economist, 1998; Knott, 1998). Although

conventional plays have been mostly identified

with successful production, Oman has added at

least one BBOE to their reserves in the past 3

years. In particular, non-associated deep-gas

production and reserve calculations from

frontier plays have exceeded industry

expectations. These reasons further justify the

need for a better understanding of the petroleum

systems of Oman and a focused and specialized

assessment of oil and gas resources in the Ghaba

and Fahud Salt Basin Provinces. The Ghaba

and Fahud Salt Basin Provinces are treated

together here since they are geologically

comparable and also both entirely within the

country of Oman.

Province Geology and Petroleum

Occurrence

Province Boundaries

The Ghaba and Fahud Salt Basin Provinces

(fig. 1) are primarily defined by their bounding

structures and, for the most part, the

geographic extent of the deep, Cambrian Ara

Salt. The Ghaba Salt Basin Province (2014) is

bounded on the east-southeast by the HuqfHaushi Uplift and outcrops, to the north by the

Oman Mountains, to the northwest by the

Central Oman Platform, and to the south and

southwest by the Central Oman High and

Ghudun-Khasfah High, respectively (fig. 2).

The Fahud Salt Basin Province (2016) is

bounded on the northeast by the Oman

Mountains, to the west by the Lekhwair-Safah

Arch, and to the south by the Central Oman

Platform. The Makarem-Mabrouk High, a

northern extension of the Central Oman

Platform (fig. 2), separates the two basins

(Gorin and others, 1982; Boote and others, 1990;

Robertson and others, 1990; Mattes and

Conway-Morris, 1990; Loosveld and others,

1996). Both the Ghaba and Fahud Salt Basin

Provinces lie entirely within the country of

Oman (see Pollastro and others, 1998).

Structural Setting

Oman is located on the southeastern margin

of the Arabian plate and is close to the

boundaries of the Iranian, Indian, and African

plates. Consequently, plate movements have

resulted in complex structural, sedimentation,

and burial histories. Oman is tectonically

bounded on the south by the Gulf of Aden

spreading zone, to the east by the Masirah

Transform Fault and the Owen Fracture Zone

Trough, and to the north by the complex

Zagros-Makran convergent plate margin,

compression along which produced the Oman

Mountains (Loosveld and others, 1996).

Precambrian metamorphic and igneous basement rocks are known from a limited number of

wells and from exposures of bedrock along the

Huqf-Haushi Uplift on Oman’s eastern margin

(figs. 1 and 2).

The Ghaba Salt Basin, South Oman Salt

Basin (2011; figs. 1, 2), and to a lesser extent,

the Fahud Salt Basin are part of a series of

subsiding rift basins stretching from India and

Pakistan across the Arabian Shield to central

Iran that formed during the Infracambrian and

Early Cambrian (about 600 to 540 Ma). (Gorin

and others, 1982; Husseini and Husseini, 1990;

Mattes and Conway-Morris, 1990). These rift

basins were formed by extension from leftlateral, strike-slip (rifting and wrenching)

movement of the Najd transform fault system

which ultimately dislocated the Arabian

plate some 300 km to the east (Schmidt and

others, 1979). Generalized cross sections across

the Ghaba and Fahud Salt Basins are shown in

figures 3 and 4.
Foreland

Bulge 

Ghudun - Khasfah High

Tertiary Basin

Maradi Fault Zone Huqf-Haushi Uplift

Suneik

0 100 

Km

Fahud

Salt Basin

Ghaba

Salt Basin

Huqf

Outcrop

South Oman Salt Basin

UNITED ARAB

EMIRATES

Arabian-Persian

Gulf

Oman Mountains

Gulf of Oman

Arabian Sea

SAUDI

ARABIA

Eastern Flank

Masirah Trough

Yibal Fahud

Natih

Barik

Al Ghuba

Saih Rawl

Mafraq

Qurn

Alam

Ghaba

N.

Saih

Nihayda

Lekhwair

Salakh Arch

Central Oman High

Makarem-Mabrouk High

Lekhwair-Safah Arch

Central Oman

Platform Rub' al Khali Basin

Dhulaima

Rima

Nimr

Marmul

Al Huwaisah

24°

22°

20°

18°

54° E. 56° E. 58° E.

EXPLANATION

Salt basin

Oil or gas field

Basement

Oman

Mountains

Figure 2. Map showing salt basins, structural elements, and major oil and gas fields of Oman.

Modified from Loosveld and others (1996).

6
Makarem-1 Saih Rawl Qarn Alam

Maradi

Fault

Dhulaima Yibal Al Bashair-1

Tertiary

Nafun

Haima

Wasia/Kahmah

Akhdar

Amin/Nimr

Ara Salt

Ara Abu Mahara

Abu Mahara

Haushi Aruma

Fahud Salt Basin Ghaba Salt Basin East Flank

Source rock

14

12

10

8

6

4

2

0

14

12

10

8

6

4

2

0

Km

0 50

Depth (km)

pC basement

Northwest Southeast

Proven gas

Oil Potential traps

EXPLANATION

Unconformity

Figure 3. Generalized northwest-southeast cross section across northern Oman and the Ghaba Salt Basin, Central Oman Platform

(Makarem High), and Fahud Salt Basin showing major oil and gas fields, proven occurrences, and potential traps (see fig. 1 for location

of line of section). See figure 5 for ages of units. Modified from Droste (1997). 

7
Haushi

Seals/shale

Lower Huqf

Lower Huqf

Ara Salt

Middle Huqf

Nimr

Amin

Miqrat ss

Mabrouk Seal

Akhdar

Sahtan/Kahmah/Wasia

Aruma

Tertiary

Barik ss Andam

Saih Rawl Qarn Alam Farha South

Safiq/Ghudun

Central Ghaba Salt Basin

EXPLANATION

Mid Al Bashair

regional seal

Km

0 5

Proven gas

Oil Potential traps

Source rock

Deformed Lower

Huqf

Depth (km)

0

2

4

6

8

10

12

West East

Unconformity

Figure 4. Generalized east-west cross section across central Ghaba Salt Basin showing major oil and gas fields, proven occurrences, and potential

traps (see fig. 1 for location of line of section). See figure 5 for ages of units. Modified from Droste (1997).

8
9

Stratigraphy

The sedimentary section in the

hydrocarbon-producing provinces of Oman is

made up of rocks ranging from Proterozoic to

Recent (Hughes-Clarke, 1988). Clastic rocks

compose most of the lower Paleozoic part of the

section, whereas the Permian through Tertiary

(fig. 5) parts of the section are predominantly

carbonate rocks and reflect climatic variations

due to Oman’s changing paleolatitude through

geologic time (Beydoun, 1991). For example,

carbonate platforms were dominant during

periods when Oman was north of lat 30° S.

Earliest sediments of Oman are a clasticcarbonate-evaporite sequence of the Huqf

Supergroup (Droste, 1997) best known from

outcrops of the Huqf-Haushi swell (figs. 1 and

2) and in the subsurface in southeast Oman

(Gorin and others, 1982; Hughes-Clarke, 1988).

The Huqf Supergroup contains several clastic

and carbonate source rocks of exceptional

quality; Huqf source rocks form the basis of the

primary petroleum systems for hydrocarbons

produced throughout Oman. The Cambrian Ara

Formation is a carbonate/evaporite sequence

with thick salt deposits (as much as 1,000 m)

(fig. 5). The thick Ara evaporites were

deposited in geographically restricted basins

during periods of low relative sea level where

stratified, anoxic conditions periodically

prevailed and organic-rich sediments and salt

were deposited (Mattes and Conway-Morris,

1990; Edgell, 1991).

The lower Paleozoic section along the

southern rim of the Arabian platform comprises

mainly continental clastics, with some marine

intercalations, which form important

hydrocarbon reservoirs in the Ghaba and

Fahud Salt Basins. A thick sequence of rift-fill

terrigenous and shallow-marine siliciclastics of

the Haima Supergroup (Droste, 1997) overlies

the Ara Formation. These sediments were

derived mainly from the south during a period

of rift-fill sagging and downloading (AlMarjeby and Nash, 1986; Sykes and Abu

Risheh, 1989; Droste, 1997). In the Ghaba Salt

Basin, sediments of the Haima Supergroup fill

and cover the margins of the basin reaching

thicknesses in excess of 6 km along the central

axis (Droste, 1997). Preexisting, highly

variable topography caused major variations in

sediment infill and syndepositional movement

of the underlying salt (halokinesis), and

differential subsidence across basement highs

influenced thickness and lateral extent of these

clastics (Aley and Nash, 1985; Heward, 1990;

Loosveld and others, 1996).

Numerous unconformities are present

throughout the Paleozoic in Oman. Several

erosional surfaces related to Ordovician

glaciation separate transgressive open-marine

to regressive deltaic cycles of the Safiq Group;

however, most of the Safiq Group and overlying

Silurian to Devonian rocks are not preserved.

In particular, two major and very broad uplift

and erosional events in eastern Oman removed

most of the Silurian and Lower Devonian

sediments and the interval between Middle

Devonian and Upper Carboniferous; these

erosional events are recognized in deep wells

from the main producing fields in the Ghaba

and Fahud Salt Basins.

Late Carboniferous time is marked in Oman

by glaciation and subsequent deposition of

glacial clastics of the Al Khlata Formation

(Levell and others, 1988) and shallow marine

and fluvial clastics of the Gharif Formation,

both of which compose the Haushi Group

(Hughes-Clark, 1988) and are important

hydrocarbon reservoirs throughout Oman.

Clastics of the Haushi Group (fig. 5), confined

mainly to interior Oman and derived from

southern sources, represent the transition from a

dominantly clastic system to a prolonged phase

of carbonate deposition.

Development of a regional shallow

carbonate platform during a middle Permian

marine transgression deposited the widespread

lower Khuff Formation (fig. 5) (Sharief, 1982),

which forms a major regional seal above the

clastic reservoirs of the Gharif Formation.

Subsequent transgressions resulted in blanket

deposits of the Jurassic to Cretaceous

(Cenomanian) sequence of mainly cyclic shelf

carbonates over northern Oman. This includes

the carbonate rocks of the Jurassic Sahtan and

the Cretaceous Kahmah (Thamama) and

Wasia Groups (fig. 5). In general, regional

changes in sedimentation during the Jurassic

and most of the Cretaceous in northern Oman

were controlled mostly by eustatic fluctuations

rather than tectonics. The youngest prograding

carbonate sequence of this succession is the
Rahab mbr

Tertiary

CENOZOIC

FARS

HADHRAMAUT

Cretaceous

MESOZOIC

KAHMAH

(Thamama)

TUWAIQ

DHRUMA

MAFRAQ

KHARAIB

SHU'AIBA

Jurassic

HAUSHI AL KHLATA

GHARIF

KHUFF

JILH

AKHDAR

Carboniferous Permian Triassic Devonian Silurian

PALEOZOIC

MISFAR

SAFIQ

GHUDAN

HUQF HAIMA

Cambrian Ordovician

MAHATAHUMAID

ANDAM

MAHWIS

MIQRAT

AMIN

ARA NAFUN

KHUFAI

AL NOOR

BIRBA

Buah

Shuram

WASIA NATIH

ARUMA

BASEMENT

Proterozoic

PRECAMBRIAN

ABU MAHARA

50

100

150

200

250

300

350

400

450

500

550

600

650

700

FIQA

AGE

(MA)

BARIK

Reservoirs

SAHTAN

Base Jurassic

Unconformity

NAHR UMR

Oil production

Future target

Gas production

Gas & condensate

Lekhwair

Miqrat/Barik

Source rocks

Sahmah

Sudair mbr

CHRONOSTRAT

GROUP or

SUPERGROUP FORMATION LITHOLOGY Ghaba Salt Basin Fahud Salt Basin

Natih

Shu'aiba

Kharaib

Khuff

Buah Buah

Natih 'e' & 'b'

sources

Gharif

Al Khlata

Mafraq

Fiqa

North

Oman

Huqf

& 'Q'

sources

North

Oman

Huqf

sources

Figure 5. Stratigraphic section of Oman showing source rocks and producing reservoirs for Ghaba and Fahud

Salt Basins. Modified from Loosveld and others (1996) and Droste (1997).

10
11

Natih Formation of latest Albian to Early

Turonian. Organic-rich marls of the Natih

Formation accumulated in intrashelf, restricted

basins during periods of global anoxic events.

A major change in tectonic style and

depositional setting took place at about the

Cenomanian-Turonian boundary mostly because

of collision and partial subduction of the

eastern Arabian sub-plate; these compressional

tectonic events are responsible for thrusting,

fore-bulge, and downwarping of a foreland

basin in northern Oman, ultimately forming the

Omani foredeep (Loosveld and others, 1996).

Flexural extension from downwarping initiated

and (or) reactivated normal faulting along a

northwest-southeast trend, such as those

associated with Natih and Fahud fields (fig.

2). Moreover, the Late Cretaceous was a period

of pronounced salt movement in the Ghaba and

Fahud Salt Basins. Salt movement is

associated with a large number of producing oil

fields, specifically in the southeastern Arabian

Gulf region in the offshore and in the onshore

Ghaba and South Oman Salt basins. This

structural event is also roughly coincident with

a worldwide eustatic sea-level rise during the

Late Cretaceous (Vail and others, 1991).

Combined, these events resulted in a change

from a shallow, stable platform to a deepwater marine environment. In the Ghaba and

Fahud Salt Basins, a significant unconformity

is present between the Wasia Group and the

overlying, deeper marine, pelagic shale and

carbonate facies of the thick Fiqa Formation

(Santonian-Campanian) of the Aruma Group

(Hughes-Clarke, 1988). Shallow-water

carbonate deposition was reestablished during

the Maastrichtian portion of the Aruma (fig.

5). Early Tertiary sediments, including

carbonates of the Hadhramaut and arid

continental clastics and marine rocks of Fars

group, disconformably overlie the Aruma.

Petroleum System Overview

Several potential petroleum source rocks

have been identified in the stratigraphic

section of Oman and range in age from

Proterozoic to mid-Tertiary; however, organicrich source rocks of Tertiary age are thermally

immature (Grantham and others, 1988; 1990).

Grantham and others (1988; 1990), and AlRuwehy and Frewin (1998) have identified

five chemically distinct types of crude oils in

Oman that are best identified by distinct

biomarkers and carbon isotope values. Four of

these oil types are linked to source rocks from

the following: (1) a broad group of oils from the

Infracambrian Huqf Supergroup, (2) the Lower

Silurian Sahmah Formation of the Safiq Group

(Droste, 1997), (3) the middle Cretaceous

Natih Formation, and (4) the Upper Jurassic

Diyab (Grantham and others, 1988; 1990) or

Tuwaiq/Hanifa equivalents (Lake, 1996; AlRuwehy and Frewin, 1998) source. A fifth type,

referred to as “Q-type” or “questionable source”

oils, has characteristics and occurrences that

suggest a separate source unit of the Huqf

Supergroup (Grantham and others, 1988, 1990)

presumably prevalent in the Ghaba Salt Basin

(Guit and others, 1995; Al-Ruwehy and Frewin,

1998; Richard and others, 1998a, b). The

unidentified “Q” source rock of the Huqf

Supergroup is tentatively interpreted as Early

Cambrian in age (Lake, 1996) and has recently

been referred to by Richard and others (1998b)

as the Dhahaban source rock inverval at the

top of the Ara Salt. Some geochemical

characteristics of these five oil types are

compared in table 1. The geographic

distribution of these oils is shown in figure 6.

Lake (1996) summarized the main source

rocks of Oman and identified seven specific

source-rock units within the Huqf Supergroup.

These source rocks or equivalents, other than

the unknown “Q” source, have been identified

from well penetrations in the South Oman Salt

Basin (2011); however, the presence of similar

source beds is predicted to occur in portions of

the Ghaba and Fahud Salt Basins. Terken

(1998; in press) differentiated the major oil

families of Oman using a cross plot of C2 7

sterane percentage versus total oil carbon

isotope value (fig. 7) . Moreover, similar cross

plots of oils sourced from Huqf rocks and oils

produced from fields of the Ghaba and Fahud

Salt Basin, referred to as North Oman Huqf

oils (fig. 7), could be distinguished from oils

sourced from Huqf rocks that were produced

from South Oman fields (South Oman Huqf

oils) (fig. 7). Therefore, based on the data of

Terken (1998; in press), the term “North Oman

Huqf” oils will be used in this study to define
Saudi Arabia

United Arab

Emirates

Oman Mountains

Arabian Sea

Natih oil (Cretaceous)

Safiq oil (Silurian)

S. Oman Huqf oil (Infracamb.)

N. Oman Huqf oil (Infracamb.)

"Q" oil (Infracamb.-Salt)

Mixed Tuwaiq/Huqf oil

Tuwaiq oil (Jurassic)

Ghaba Salt

Basin Province

Fahud Salt

Basin Province

0 100 KM

"Q" oil

migration

fairway

Figure 6. Geographic distribution of oil types found in Oman and approximate “Q” oil migration

fairway. Data compiled from numerous sources including Grantham and others (1988), Sykes and

Abu Risheh (1989), Guit and others (1995), Lake (1996), Al-Ruwehy and Frewin (1998), Richard and

others (1998), and Terken (1998, in press). Not all oils shown here are discussed in this report. Solid

green dots indicate oil fields; solid red dots indicate gas fields.

55° 56° E. 57° E. 58° E. 59° E. E.

21°

23°

18°

22°

20°

19°

17°

12
0 10 20 30 40 50 60 70 80 90 100

C27 STERANE, IN PERCENT

South Oman Huqf Oils

North Oman Huqf Oils

"Q" Oils

TOTAL 

C13 OIL ISOTOPE

-27.5

-30.0

-37.5

-35.0

-32.5

-25.0

Tuwaiq Oils

Natih Oils

Mixtures

Figure 7. Various populations for Oman oil types identified from cross plot of total carbon isotope

versus C27 sterane percent. Modified from Terken (in press).

13
14

the Total Petroleum Systems of the Ghaba and

Fahud Salt Basin Provinces.

Hydrocarbon accumulations have been

recognized in carbonate and clastic units

throughout the stratigraphic section of the

Ghaba and Fahud Salt Basins, but mostly

within reservoirs of Cretaceous, CarboniferousPermian, and Cambrian-Ordovician age

because of their proximity to excellent

overlying seals. Major accumulations and plays

have been identified in the Infracambrian Huqf

Supergroup of the South Oman Salt Basin

(2011) where these units can be reached by

drilling (Al-Marjeby and Nash, 1986; Sykes

and Abu Risheh, 1989; Mattes and ConwayMorris, 1990; Boserio and others, 1995; Hartstra

and Graham, 1996; Lake, 1996; Onderwaater

and others, 1996; Amthor and others, 1998).

Recently, potential for Huqf deep-gas in Buah

Formation carbonates was revealed in the

Makarem 1 (ST-2) well along the MakaremMabrouk High, Central Oman Platform

Province of north-central Oman, suggesting

further potential for other pre-salt Huqf deepgas targets in the Ghaba and Fahud Salt Basin

Provinces, particularly around salt diapirs.

The primary reservoirs of the Ghaba and

Fahud Salt and their relative stratigraphic

distribution and types of hydrocarbons

produced are illustrated in the stratigraphic

section of figure 5. Reservoirs and associated

seal rocks are also summarized in the Total

Petroleum Systems Events charts of figures 8

through 10.

Traps of the Ghaba and Fahud Salt Basin

Provinces are mainly structural in origin and

were formed by one or more mechanisms during

periodic halokinesis of the thick Cambrian

Ara Salt and from consequent folding and

faulting from basin loading, rifting, or other

major tectonic events. This interplay of

tectonics and salt movement was progressive,

constantly modifying basin architecture,

controlling sedimentation, and more crucially,

modifying trap geometries (Brannan and

Flanagan, 1998), particularly in the Late

Cretaceous and late Tertiary in areas where

the Oman Mountains and associated forelandbasin system developed. Most of the future

new-field discoveries will be focused on lowrelief structures, as many of the large structures

have been drilled. In contrast, although the

crests of domes and anticlines above salt

diapirs have been major exploration targets,

salt-diapir flank traps, known to be significant

exploration targets elsewhere in the world,

have gone essentially untested (Faulkner,

1998).

Few pure stratigraphic traps have been

identified in the fields of the Ghaba and

Fahud Salt Basins. Recently, however,

Richard and others (1998a, b) have suggested

that exploring for stratigraphic traps along the

extensive “Q”-oil migration fairway (fig. 6)

has the greatest potential for new Haushi

reservoirs in North Oman. Although most

fields in the Ghaba and Fahud Salt Basins are

of structural origin, stratigraphic trapping

elements are commonly involved in the

formation of the accumulation. For example,

accumulations in a large Cretaceous rudistid

complex with a strong structural overprint in

Shu’aiba reservoirs at Al Huwaisah field,

Fahud Salt Basin, play a key role in

productivity (Vahrenkamp and Grotsch, 1995).

Moreover, opportunity for identifying pure- or

combination-stratigraphic traps is particularly

increased as improved technology and

subsurface imaging provide new, higher

resolution data.

Hydrocarbon Exploration and

Production History

Oil exploration in Oman began in the 1920’s

with the first exporation license awarded for a

2-year period to D’Arcy Exploration Company

in 1925; however, the first discovery was not

made until 1962. A concession was granted to

Petroleum Development Limited (PDL) of

Oman and Dhofar, an affiliate of the Iraq

Petroleum Company (IPC) and Associated

Companies, in 1937 covering Oman territory.

The outbreak of WWII interupted exporation

activities. In the late 1940’s, exploration

resumed with reconnaissance studies in Dhofar.

PDL relinquished the Dhofar concession and

changed its name to Petroleum Development

Oman (PDO). Four exploratory wells were

drilled between 1956 and 1960, including the

Fahud-1 well, which missed the billion barrel

Fahud field by less than 500 m (Graham and

Naylor, 1996).
PRESERVATION

CRITICAL MOMENT

GENERATIONTRAP FORMATION

OVERBURDEN ROCK

RESERVOIR ROCK

SEAL ROCK

SOURCE ROCK

ROCK UNIT

PETROLEUM

SYSTEM EVENTS

GEOLOGIC 

TIME

SCALE MESOZOIC CEN.

TR TERT

E. M. L. E. M. L. E. L. E. M. L. E. L. E. L. L. E. M. L. E. L. E.

JUR CRET

MIGRATIONACCUMULATION

PALEOZOIC

PreC CAM ORD SIL DEV M P P

O M

 600 550 500 450 400 350 300 250 200 150 100 50 0

Province Name:_________ TPS Name: ______________________

Author(s):___________ Date:_____________

Mabrouk Mbr

Al

Bashair

Mbr

Miqrat & Amin Fms,

Barik ss Mbr

Haima

Supergroup

Huqf

Supergroup

Haushi

Group

Al Khlata Fm Gharif Fm

Kahmah

(Thamama)

Group

Wasia

Group

Shu'aiba &

Kharaib Fms

Natih Fm

Nahr Umr Fm Fiqa Fm

2014 -- Ghaba Salt Basin

R.M. Pollastro

201401 -- North Oman Huqf/"Q" - Haushi(!) 

Rahab Shale Haushi Ls

Khuff Fm

Abu Mahara, Shuram,

Buah,("Q"-source?), & Ara Fms

Uplift & erosion

Total Petroleum System

Events Chart

8/11/98

Halokinesis Tilting & doming

accompanied by halokinesis

1st & 2nd

Alpine events

Figure 8. Petroleum system events chart for North Oman Huqf/“Q” -- Haushi(!) Total Petroleum System (201401), Ghaba Salt Basin

Province (2014), Oman.

Early Huqf/"Q" oil & gas

generation

Main phase Huqf

gas generation

?

Ara &

Buah (?)

Ara salt &

Nimr shale

"Q" oil

migration

Main phase "Q" &

Huqf oil generation

Huqf oil

migration

15
PRESERVATION

CRITICAL MOMENT

GENERATIONTRAP FORMATION

OVERBURDEN ROCK

RESERVOIR ROCK

SEAL ROCK

SOURCE ROCK

ROCK UNIT

PETROLEUM

SYSTEM EVENTS

GEOLOGIC 

TIME

SCALE MESOZOIC CEN.

TR TERT

E. M. L. E. M. L. E. L. E. M. L. E. L. E. L. L. E. M. L. E. L. E.

JUR CRET

MIGRATIONACCUMULATION

PALEOZOIC

PreC CAM ORD SIL DEV M P P

O M

600 550 500 450 400 350 300 250 200 150 100 50 0

Province Name:_________ TPS Name: ______________________

Author(s):___________ Date:_____________

Mabrouk Mbr

Al

Bashair

Mbr

Miqrat & Amin Fms,

Barik ss Mbr

Haima

Supergroup

Huqf

Supergroup

Haushi

Group

Al Khlata Fm Gharif Fm

Kahmah

(Thamama)

Group

Wasia

Group

Shu'aiba &

Kharaib Fms

Natih Fm

Nahr Umr Fm Fiqa Fm

Fahud Salt Basin (2016)

R.M. Pollastro

North Oman Huqf - Shu'aiba(!)

Rahab Shale

Abu Mahara, Shuram,

Buah, &Ara Fms

Uplift & erosion

Ara &

Buah (?)

Ara Salt &

Nimr Shale

Figure 9. Petroleum system events chart for North Oman Huqf -- Shu’aiba(!) Total Petroleum System (201601), Fahud Salt Basin Province (2016), Oman.

8/19/98

Halokinesis

Early Huqf

oil generation

1st & 2nd

Alpine events Intermittent halokinesis & fault-growth

Main stage Huqf

gas generation

Main stage Huqf

oil generation

Total Petroleum System

Events Chart

Haushi Ls

Khuff Fm

16
PRESERVATION

CRITICAL MOMENT

GENERATIONTRAP FORMATION

OVERBURDEN ROCK

RESERVOIR ROCK

SEAL ROCK

SOURCE ROCK

ROCK UNIT

PETROLEUM

SYSTEM EVENTS

GEOLOGIC 

TIME

SCALE

PALEO. EOCENE OLIG. MIOCENE PP

MESOZOIC CENOZOIC

TR TERTIARY

E. M. L. E. M. L. E. L.

JURASSIC CRETACEOUS QUAT.

MIGRATIONACCUMULATION

250 200 150 100 75 70 60 50 40 30 20 10 0

Wasia

Group

Natih

Fm Natih 'b' & 'e'

Nahr Umr Fm Fiqa Fm

Fiqa Fm

Total Petroleum System

Events Chart

Province Name:_________ TPS Name: ______________________

Author(s):___________ Date:_____________

Omani foredeep North-Central Fahud Salt Basin

Fahud Salt Basin (2016) Natih(!) Petroleum System

R.M. Pollastro 8/11/98

? ?

Shu'aiba

Fm

First Alpine

Event

Second Alpine

Event

Continent-continent collision and uplift of Oman Mountains.

Reactivation and inversion of normal and strike-slip faults (Natih, Maradi, and Fahud).

Some salt- assisted uplift and halokinesis forming some 20 diapirs.

Peak oil-generation of Natih source beds

Figure 10. Petroleum system events chart for middle Cretaceous Natih(!) Total Petroleum System (201602), Fahud Salt Basin Province (2016), Oman

17
18

Oil production grew from 300,000 barrels

per day (bbl/d) in 1967 through a plateau of

about 500,000 bbl/d in the 1970’s and early

1980’s. During early 1998, the average daily

production of oil in Oman was maximized to

about 900,000 bbl/d (U.S. Energy Information

Administration, 1998; Petroleum Economist,

1998; Knott, 1998); about 95 percent, or about

850,000 bbl/d, is produced by the PDO

consortium. Average oil production rate per

well is about 400 bbl/d, atypical in comparison

to that of about 6,000 bbl/d for other major

Middle East countries. Moreover, associated

produced waters are high, averaging about 2

million bbl/d in 1997, and disposal of the water

presents further complications and

consideration. Total exports are about 470,000

bbl/d with an average API density of 38°–39°,

which is lighter than the common “Oman

Blend” of about 34° API gravity (Petroleum

Economist, 1998).

Oman’s interest in the exploration for gas

has been a relatively recent development;

potential recoverable reserves predicted in 1997

are 30 trillion cubic feet (tcf) in central Oman

with more than 10 tcf located in deep geologic

structures beneath active oil fields (U.S. Energy

Information Administration, 1998). Pipelines

from Yibal to Muscat to Sohar carry the current

gas production from 13 wells. PDO has

estimated that 45 wells will be in production by

the year 2000, and gas production is predicted

to peak in 2003 and continue through 2025 to

meet contract requirements (Petroleum

Economist, 1998). Gas production and

operations will be centered around three major

fields of the Ghaba Salt Basin: Barik, Saih

Rawl, and Saih Nihayda.

Total Petroleum Systems and

Assessment Units

Ghaba Salt Basin Province (2014)

North Oman Huqf/“Q”–Haushi(!) Total

Petroleum System (201401)

The North Oman Huqf/”Q”–Haushi(!)

Total Petroleum System is interpreted here as

the only significant petroleum system of the

Ghaba Salt Basin Province in north-central

Oman. The first half of the Total Petroleum

System (TPS) name implies a combination of all

source beds of the Huqf Supergroup in the

Ghaba Salt Basin Province. This group of

source beds is referred to in this report as those

units which generate the “North Oman Huqf”-

and “Q”-type oils, as defined by the

stratigraphic and geochemical data of

Grantham and others (1988), Sykes and Abu

Risheh (1989), Richard and others (1998a, b),

and Terken (1998; in press). The second half of

the system name refers to the Haushi Group,

which are the primary reservoirs that produce

hydrocarbons from this system. Although

numerous reservoirs of various age and

lithology are included in this system,

approximately two-thirds of the fields produce

from the clastic reservoirs of the CarboniferousPermian Haushi Group. The North Oman

Huqf/“Q”–Haushi(!) TPS is summarized in the

events chart of figure 8.

Although most of the hydrocarbons

produced from the Huqf source rocks are

developed only within the salt basins or

within the salt itself (for example, the

unidentified “Q”-source rock), some source rocks

of the North Oman Huqf/“Q”–Haushi(!) TPS

are developed locally within Huqf basinal

lows (Sykes and Abu Risheh, 1989; Visser, 1991;

Lake, 1996; Richard and others, 1998a, b). The

geographic extent of (1) the pod of active source

rock, (2) minimum petroleum system, and (3)

maximum or Total Petroleum System for the

North Oman Huqf/“Q”–Haushi(!) TPS are

shown in figure 11. Note that the North Oman

Huqf/“Q”–Haushi(!) TPS extends beyond the

the Ghaba Salt Basin Province and diagonally

across the central portion of the MakaremMabrouk high portion of the Central Oman

Platform Province (2015), into the eastern flank

portion of the Rub’ al Khali Basin Province

(2019 ), and southward over the Central Oman

High and into the South Oman Salt Basin

Province (2011). Thus, the boundary over the

Makarem-Mabrouk high portion of the Central

Oman Platform Province (2015) separates fields

that are charged by “Q” and North Oman Huqf

hydrocarbons generated mostly in the Ghaba

Salt Basin proper from fields charged by

hydrocarbons generated in the Fahud Salt

Basin proper. The minimum petroleum system

boundary is defined by known occurrence and
Arabian Sea

Gulf of Oman

55° 56° 57° 58°

South Oman

Salt Basin

(2011)

Ghaba Salt Basin

(2014)

Fahud Salt Basin

(2016)

Huqf- Haushi Uplift

(2013)

Maradi Fault Zone

Rub' al Khali Basin

(2019)

East Flank Oman Sub-basin

(2012)

Oman Mountains

(2017)

Gulf of Oman Basin

(2018)

55° E. 56° E. 57° E. 58° E 59° E

21°

23°

24°

22°

20°

19°

Saudi

Arabia

U.A.E.

Oil or gas

field centerpoint

(2014)

USGS Province

boundary

USGS province number USGS assessment unit number

Assessment unit and Total Petroleum System (TPS) boundary

Foreland

Bulge

Pod of active

source rock boundary

Minimum Petroleum

System boundary

Country

boundary

0 100 km

EXPLANATION

Fahud-Natih

faults

Figure 11. Map showing Ghaba Salt Basin Province (2014), North Oman Huqf/“Q” -- Haushi(!)

Total Petroleum System (201401), and Ghaba-Makarem Combined Structural Assessment Unit

(20140101). Oil and gas field centerpoints (Petroconsultants, 1996) and boundaries for pod of active

source rock and minimum petroleum system are also shown. Scale = 1:3,600,000.

Central Oman Platform

(2015)

20140101

20140101

North Oman Huqf/'Q' - Haushi TPS

Ghudan-Khasfah

Flank (2010)

[201601] USGS TPS number

19
20

distribution of North Oman Huqf-type and (or)

“Q”-type oils (Grantham and others, 1988;

Sykes and Abu Risheh, 1989; Guit and others,

1995; Al-Ruwehy and Frewin, 1998; Richard

and others, 1998a, b; and Terken, 1998; in press)

in fields or wells throughout the area and

interpreted here as sourced from Huqf rock units

deposited within or associated with the

greater Ghaba Salt Basin proper. All known

oil and gas fields that occur within the

minimum petroleum system boundary for the

North Oman Huqf/“Q”–Haushi(!) TPS are

listed in table 2. The boundary for the Total

Petroleum System defines the maximum areal

extent of occurrence of genetically related,

Ghaba Salt Basin, Huqf- and “Q”-sourced

hydrocarbons. Depth to Huqf source rocks

within the TPS boundary likely ranges from

about 4,000 m to >10,000 m.

Reservoirs, Seals, and Trap Styles

Names and ages of reservoirs of the North

Oman Huqf/“Q”–Haushi(!) TPS and

corresponding seals and hydrocarbons produced

are summarized in figures 5 and 8. The primary

reservoirs are two formations of the middle

Carboniferous to Lower Permian Haushi

Supergroup—the Al Khlata and the Gharif.

The glacio-lacustrine Al Khlata Formation

consists of laterally continuous sandstones,

intercalated with thin shale layers which

have no sealing capacity (Levell and others,

1988). The regionally deposited Rahab Shale

member at the top of the Al Khlata

stratigraphic boundary and between the Al

Khlata and Gharif Formations commonly forms

a seal for Al Khlata reservoirs (fig. 5). In most

fields, however, the Al Khlata Formation and

laterally continuous, porous deltaic sands of the

Lower Gharif form one continuous reservoir and

are sealed by 20 to 30 m of the Haushi

Limestone member (Guit and others, 1995;

Nederlof and others, 1995). Middle Gharif

reservoirs are composed of fine-grained

sandstones, siltstones, and shales with up to 20

percent porosity but with low permeability and

are sealed with a thick (about 90 m) claystone.

Alluvial, mostly laterally discontinuous

channel sands of the Upper Gharif are sealed

by basal carbonates of the lower Khuff

Formation (Guit and others, 1995; Nederlof and

others, 1995).

Other major reservoirs are porous, commonly

fractured grainstones and chalky carbonates of

the middle Cretaceous Natih Formation (Fiqa

Formation shale seal) and Lower Cretaceous

Shu’aiba Formation (Nahr Umr Formation

shale seal) (fig. 5). Sandstones of the Barik

sandstone member (Mabrouk Shale seal) and

Miqrat Formations (Al Bashair Shale seal),

Haima Supergroup (Lower Cambrian to Lower

Ordovician) are primarily deep gas reservoirs;

potential for deep gas is also being explored in

Precambrian carbonates of the Buah Formation,

Huqf Supergroup (fig. 5).

Most of the fields of the Ghaba Salt Basin,

particularly those producing from Haushi

reservoirs, are structurally complex, saltinduced anticlines and domes that have been

broken up into several fault blocks by crestal

collapse features (Nederlof and others, 1995).

Specific common trap styles are faulted

closures, dip closures, and faulted-dip closures

(Lake, 1996).

Source Rock Character and

Geochemistry

Infracambrian Huqf source rocks contain

structureless, type I and type II oil-prone

organic matter (Grantham and others, 1988,

1990; Mattes and Conway-Morris, 1990; Lake,

1996). Huqf source rocks produce a rather broad

group of North Oman Huqf-type oils, which

are recognized by having high abundances of of

C29 steranes (greater than 50 percent) relative

to other steranes and ∂13C measured at –37.1‰.

South Oman Huqf-sourced oils can be

distinguished from North Oman Huqf-sourced

oils in cross-plots of isotopic and C27 sterane

percent compositions (fig. 7). North Oman

Huqf oils have ∂13C compositions between

–31‰ and –35‰ and C27 sterane as high as 45

or 50 percent, whereas South Oman Huqf oils

are between –35‰ and –37‰ and C27 sterane

<25 percent. Richard and others (1998a, b)

have identified the Shuram Formation of the

Huqf Supergroup as the primary source of Huqf

oils in Haushi reservoirs of North Oman. The

Shuram contains carbonate source rocks that are
21

thick (about 450 m), laterally extensive, and

average about 2 percent TOC (Lake, 1996).

The main geochemical characteristics of the

Huqf-type oils, which correlate well with

extracts from Huqf source rocks, are high sulfur

(1.5–2.0 percent) content, the presence of a

homologous series of long-chain, methylsubstituted alkanes, the so-called “X”-

branched compounds (Klomp, 1986), high (45–

90 percent) C27 sterane percent, and highnegative (–28 to –33‰) carbon isotope. The

geochemical characteristics of North Oman

Huqf-type and “Q”-type oils are summarized in

table 1.

The “Q”-type oils were first identified by

Grantham and others (1988) from fields in

central Oman and later by Sykes and Abu

Risheh (1989), Guit and others (1995), AlRuwehy and Frewin (1998), and Richard and

others (1998a, b) in more northern fields. These

commonly light (average API gravities of about

40), relatively low sulfur (about 0.2 percent),

mature oils are found in Paleozoic clastic

reservoirs and occur where Huqf Group rocks are

deeply buried, at depths greater than those

routinely penetrated by drilling, and

unsampled. The “Q” oils also contain the socalled “X-compounds,” evidence which argues

for the presumed Infracambrian age of the “Q”

source rock. Grantham and others (1990) and

Guit and others (1995) speculated that the

distinctive “Q” oil is from a source rock in an

undrilled Huqf level, had a specific flourishing

biomass, contained type I and type II organic

matter, and is present in a limited extent

within the salt in the Ghaba Salt Basin.

Other specific source rock characteristics, such

as thickness and TOC, of the “Q” source rock(s)

are still unknown. The “Q”-sourced oils are

recognized by having a high (>50 percent) total

C27 sterane abundance relative to other steranes

and ∂13C measured at –30.4‰. Additionally,

the “Q” oils have a distinctive compound not

found in other oils and yet to be identified in a

known source rock (Al-Ruwehy and Frewin,

1998). Pure “Q” oil end members have been

identified in Haima and Natih reservoirs in

Saih Rawl and Bahja fields.

Most oil accumulations in Central Oman are

probably mixtures from both Huqf and “Q”

source rocks. Commonly, “Q” oils are found in

continuous strings of accumulations below the

regional Khuff seal on northerly plunging

structural highs. The Huqf oils, however, are

found at all stratigraphic levels in structures

related to salt domes and fault zones (Guit and

others, 1995), suggesting a vertical mechanism

for Huqf hydrocarbons as opposed to a

predominantly horizontal migration for “Q”

oils.

Burial History, Generation, and Migration

Generation of hydrocarbons from Huqf

source rocks in North and Central Oman has

been discussed by Sykes and Abu Risheh (1989),

Visser (1991), Guit and others (1995),

Borgomano and others (1996), Droste (1997),

and Amthor and others (1998). Burial history

curves for the North Oman Huqf source rocks in

the Ghaba Salt Basin Province are shown in

figures 12 and 13.

Vitrinite reflectance equivalent (VRE) of

Huqf source rocks within the North Oman

Huqf/“Q”–Haushi(!) TPS ranges from about 0.6

percent to 4.0 percent (Sykes and Abu Risheh,

1989; and Visser (1991). Basin modeling by

Amthor and others (1998) suggests three

periods of hydrocarbon generation in the Ghaba

Salt Basin: (1) during Haima (Andam-Safiq)

deposition (about 520–373 Ma); (2) during

Akhdar-Kahmah deposition (270–100 Ma);

and (3) during Aruma/Tertiary deposition (80–0

Ma). Deep reservoirs of the Haima Supergroup

were charged with oil mainly during the second

stage. Gas was generated by Huqf sources as

early as 510–375 Ma in the central Ghaba Salt

Basin. Gas generated during the second stage

was sourced on the west flank of the Ghaba

Salt Basin and migrated mainly east into

Haima structures along the western margin of

the basin. Down-building of salt in the deeper

central Ghaba Salt Basin inhibited migration

of the western flank gas charge from reaching

structures along the eastern margin of the basin.

Stratigraphic distribution of the Huqf

source rocks relative to the Ara Salt (that is,

pre-salt, intra-salt, post-salt) and salt

movement also play a critical role in the

migration and charging of reservoirs. For

example, modeling by Borgomano and others

(1996) in the Ghaba Salt Basin shows at least

two stages of Huqf oil generation and reservoir
VRE = 0.62 

VRE = 1.20

VRE = 2.40

670 600 505 438 408 360 286 248 213 144 65 2

Precambrian Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Tertiary

Ghaba Salt Basin

6000

7000

8000

9000

10000

4000

8000

3100

2766

2066

1813

1385

 905

 429

9000

9500

5000

3000

2000

1000

0

Depth (meters)

m.y.

Generation -- Middle Huqf

HC expelled

M. Huqf

oil

U. Huqf oil U. Huqf gas

M. Huqf

gas

Cumulative

Upper Huqf

Middle Huqf

Upper Huqf

Middle Huqf

Figure 12. Burial-history diagram, vitrinite reflectance equivalent (VRE), and stages of hydrocarbon (HC) generation for Huqf

source rocks from unidentified well in vicinity of Saih Rawl field, Ghaba Salt Basin Province, Oman. Modified from Visser (1991).

22
610 570 510 476 439 409 386 363 323 290 256 208 178 148 97 65 35 5 Ma

6000

5000

2000

1000

3000

4000

0

Depth (meters)

Barik Field Burial History

140 °C

120 °C

80 °C

100 °C

Barik Sandstone Mbr

Huqf intra-salt

oil generation

Release, migration, & charge

 of Huqf intra-salt oil

Huqf

gas charge

& oil flushing

Figure 13. Burial-history diagram for deep-gas reservoirs, Barik Sandstone Member, showing temperatures and stages of oil

and gas charge from Huqf intra-salt source rocks, Barik field, Ghaba Salt Basin. Modified from Borgomano and others (1996).

23
24

charge with subsequent Huqf gas generation and

gas flushing of the second stage of oil charge.

“Q” oils are speculated to be sourced within

the Ghaba Salt Basin (Grantham and others,

1988; Sykes and Abu Risheh, 1989; Visser, 1991;

Guit and others, 1995; Richard and others,

1998a). However, Richard and others (1998b)

more recently suggested from seismic data that

the “Q” oils may in fact be derived from two

different “kitchens”: (1) a source along the

western margin of the Ghaba Salt Basin, and

(2) a possible source in the south Fahud Salt

Basin. In these presumed “Q”-source kitchens,

“Q” charge appears to postdate Huqf charge

with Huqf oils generated as early as

Ordovician (Visser, 1991). Older structures in

the Ghaba Salt Basin and closest to the “Q”

source rock seem to have been flushed later

with light “Q” oil with lateral migration

along north-south-trending structural highs.

Migration of “Q” oils was thus through the

Gharif section in a southerly direction

gradually moving into stratigraphically

younger units and over a distance of more than

150 km (Guit and others, 1995). Moreover,

faulting during latest Tertiary caused

breaching of the main Khuff seal, allowing

vertical migration of Huqf oils into younger,

post-Upper Cretaceous structures. Thus, pure

Huqf-type oils are found in fault-related

structures where pathways for the lateral

migration of “Q” oils have been blocked. The

geographic distribution of known Huqf and “Q”

oil accumulations, including admixtures of both,

is shown in figure 6.

Oil typing in northern Oman shows that

over 90 percent of oil in place in Haushi

reservoirs is derived from the “Q” source rock.

The “Q” kitchen was defined using chemical

“odometers” (mainly nitrogen) that provide a

measure of migration distance (Richard and

others, 1998a, b). Chemical odometer tracing

from 18 “Q” oils suggests that the “Q” source

may be located on the western margin of the

Ghaba Salt Basin, in the vicinity east of Saih

Rawl field (fig. 2). Seismic lines across this

area show salt-filled rim synclines that

possibly contain the “Q” source beds. Also, the

second possible source area in the Fahud Salt

Basin is marked on seismic lines with a highamplitude reflector at the top of the Ara Salt

(Richard and others, 1998b). Both source areas

are considered to have been restricted areas

during Ara deposition and ideal sites for sourcerock preservation. The high efficiency of the

main overlying seal, the Khuff Formation, has

allowed for long-distance migration of “Q” oils,

as demonstrated by chemical odometers, and

the new-field discoveries along the “Q”-

migration fairway (fig. 6).

Modeling of hydrocarbon generation for the

top-salt Dhahaban (“Q”) source-rock interval

by Richard and others (1998b) indicates that

“Q” oil was generated in the Ghaba Salt Basin

source area in the Paleozoic and Mesozoic and

peaked in the middle Paleozoic and Triassic. In

the shallower southernmost portion of the

Fahud Salt Basin to the west, “Q” oil

generation occurred from the Jurassic until early

Tertiary peaking twice during the Early and

Late Cretaceous.

Biodegraded, low API (<20°) gravity oils

are produced from Haushi and Haima Group

reservoirs in an area along the east-northeast

flank of the Ghaba Salt Basin. Al Lamki and

Terken (1996) have shown that these reservoirs

are also major aquifers and are in hydraulic

communication. Hydrodynamic activity in the

Ghaba Salt Basin has reduced temperatures

and salinities of formation waters where

recharge has occurred over the history of the

basin, degrading the oils. Recharge of meteoric

waters produces a geochemical environment

conducive for biodegradation by oil-reducing

bacteria in hydrocarbon-bearing reservoirs by

reducing reservoir temperatures (and

geothermal gradient) and carrying oxygen and

nutrients. Moreover, areas of highly mature

oils in the Ghaba Salt Basin also define the

southward-migrating “Q”-oil fairway (fig. 6)

that was not affected by biodegradation.

Ghaba-Makarem Combined Structural

Assessment Unit (20140101)

One all-inclusive assessment unit, designated as the Ghaba-Makarem Combined

Structural Assessment Unit, has been assigned

to the North Oman Huqf/“Q”–Haushi(!) TPS

of the Ghaba Salt Basin Province. The

assessment unit boundary is defined and

described by the TPS boundary and includes all

reservoirs (Haima, Haushi, Shu’aiba, Natih,
25

among others) and styles of structural traps

(domes, anticlines, fault blocks, for example) of

the known inclusive fields (fig. 8; table 2).

Thus, the Ghaba-Makarem Combined

Structural Assessment Unit boundary outlines

the maximum geographic extent at which

similar undiscovered fields may exist in the

North Oman Huqf/“Q”–Haushi(!) TPS (fig.

11).

Expected exploration strategies and areas of

new-field discoveries in this assessment unit

are:

1. new-field discoveries for deep gas/condensate (>5,000 m) in clastic reservoirs of the

Haima Supergroup and possibly deeper

limestone reservoirs of the Buah Formation,

Huqf Supergroup, in north-central Oman.

2. growth of proven gas/condensate

reserves from Haima Supergroup in existing

fields (field growth).

3. salt diapir flank traps in Natih,

Shu’aiba, Haushi, and Haima targets (see

Faulkner, 1998).

4. new fields in Haushi (Al Khlata and

Gharif) clastics, particularly within the “Q”

oil migration fairway (fig. 6).

5. new fields in Shu’aiba and Natih

limestones in low-relief, fractured “pancake”

structures (large structures already drilled).

6. growth of proven oil reserves in existing

fields (field growth).

Limitations, conditions, and risks that will

affect areas of exploration, particular plays,

and sizes and numbers of fields include:

1. the occurrence of pore-plugging pyrobitumen in Haima reservoirs in areas along

Makarem-Mabrouk high.

2. low (15°–20°) API, biodegraded oils in

Haushi and Haima reservoirs in areas close to

meteoric recharge, particularly along eastsoutheast flank of the Ghaba Salt Basin and in

Huqf-Haushi Uplift Province (2013).

3. substantial risk of low-quality

(nitrogen-rich) gas along Oman Mountain front

and overthrust.

It is predicted in this study that a large

portion of the exploration efforts in northcentral Oman will be focused on deep

gas/condensate resources in Haima, and

possibly Huqf, reservoirs. Because much of the

gas resource is likely to be discovered in

existing fields, a large gas growth factor, the

Mid-Continent growth factor derived from the

U.S. Geological Survey 1995 National Oil and

Gas Assessment, was used in the assessment of

resources.

Fahud Salt Basin Province (2016)

North Oman Huqf–Shu’aiba(!) Total

Petroleum System (201601)

The North Oman Huqf–Shu’aiba(!) Total

Petroleum System is interpreted here as the

dominant petroleum system of the Fahud Salt

Basin Province of North Oman and overlaps, in

part, with the middle Cretaceous Natih(!) TPS

discussed in the following section. The first

half of the TPS name implies a combination of

all source beds of the Huqf Supergroup in the

Fahud Salt Basin Province that generate the

“pure Huqf-type” oils, referred to here as

“North Oman Huqf” -type, as defined by the

stratigraphic and geochemical data of

Grantham and others (1988), Sykes and Abu

Risheh (1989), Richard and others (1998a, b),

and Terken (1998; in press). The second half of

the system name refers to the carbonate

reservoirs (porous rudist buildups and fractured

chalk) of the Cretaceous Shu’aiba Formation,

which to date have produced most of the

hydrocarbons generated from this system (for

example, Yibal field, Knott, 1998). Similar to

the North Oman Huqf/“Q”–Haushi(!) TPS,

numerous reservoirs of various age and

lithology are included in this system and are

summarized in figure 5. Approximately onethird of the fields produce from Shu’aiba

reservoirs; however, in-place reserves from the

high-porosity, low-permeability, fractured

Shu’aiba at Yibal field alone are estimated at

about 3 billion barrels of “stocktank oil”

(Alsharan and Nairn, 1997). The North Oman

Huqf–Shu’aiba(!) TPS is summarized in the

events chart of figure 9.

Most of the hydrocarbons produced from the

Huqf source rocks are developed within the

Infracambrian salt basin with some source rocks

developed locally within Huqf basinal lows

(Sykes and Abu Risheh, 1989; Visser, 1991;

Lake, 1996; Richard and others, 1998a, b). The

geographic extent of (1) the pod of active source

rock, (2) minimum petroleum system, and (3)
26

maximum or Total Petroleum System for the

North Oman Huqf–Shu’aiba(!) TPS all

coincide and are shown in figure 14. The North

Oman Huqf–Shu’aiba(!) TPS is interpreted

here to extend beyond the Fahud Salt Basin

Province boundary and onto the central portion

of the Makarem-Mabrouk high of the Central

Oman Platform Province (2015). A small

portion of the system also extends into the

eastern flank portion of the Rub’ al Khali

Province (2019). Thus, the boundary onto the

Makarem-Mabrouk high, Central Oman

Platform Province, indicates that some fields,

particularly deep gas fields, are charged from

Huqf hydrocarbons generated within the

Fahud Salt Basin (Amthor and others, 1998).

The minimum petroleum system boundary is

defined by the known occurrence and

distribution of North Oman Huqf-type oils

(Grantham and others, 1988; Sykes and Abu

Risheh, 1989; Guit and others, 1995; AlRuwehy and Frewin, 1998; Richard and others,

1998a, b; and Terken, 1998; in press) in fields or

wells throughout the area and interpreted here

as sourced from Huqf rock units deposited

within, or in association with, the greater

Fahud Salt Basin proper. All known oil and

gas fields that occur within the minimum

petroleum system boundary for the North Oman

Huqf–Shu’aiba(!) TPS are listed in table 2.

The Total Petroleum System boundary for the

North Oman Huqf–Shu’aiba(!) TPS is defined

as the maximum areal extent of hydrocarbons

generated by Huqf source rocks of the Fahud

Salt Basin proper. The estimated range of

depth to Huqf source rocks within the TPS

boundary interpreted from the published

structure contour maps of Sykes and Abu Risheh

(1989) is 5,000 m to >8,000 m.

Reservoirs, Seals, and Trap Styles

Names and ages of reservoirs of the North

Oman Huqf–Shu’aiba(!) TPS and corresponding seals and hydrocarbons produced are

summarized in figures 5 and 9. Over 90 percent

of the fields in the Fahud Salt Basin, half of

which are gas fields, produce from the highporosity, commonly fractured grainstones and

chalky carbonates of the Lower Cretaceous

Shu’aiba Formation and middle Cretaceous

Natih Formation. Although some fields

producing from Natih reservoirs are sourced by

the organic-rich facies of the Natih Formation

(the Natih(!) Total Petroleum System

discussed later), a large volume of oil and gas

in Natih reservoirs is sourced from the Huqf.

Most production, however, is from the Shu’aiba

Formation (Thamama-Khamah Group, Early

Aptian) (Murris, 1980; Alsharhan and Nairn,

1997). The Shu’aiba is overlain by the Nahr

Umr Formation of the Wasia Group, a

widespread transgressive shale, which forms a

regional seal for these reservoirs (fig. 5). The

integrity of this seal is excellent, as

hydrocarbons produced from the Shu’aiba are

commonly different from those produced in

middle Cretaceous Natih reservoirs overlying

the Nahr Umr shales (Brennan, 1985). In both

the Shu’aiba and Natih Formations, shallowwater, shelf-margin carbonate buildups

(mainly rudistid reefs) and associated

grainstones (debris shoals) formed on and

around low-relief structural highs (mostly

formed by salt pillows and tilted, upthrown

fault blocks) comprise the best reservoirs (Frost

and others, 1983; Harris and Frost, 1984).

Uplift from both tectonic and halokinetic

movements produced secondary (mostly moldic

and vuggy) porosity from subaerial erosion and

meteoric diagenesis. Porosities ranging from 30

to 40 percent have been recorded in the

Shu’aiba at Yibal, Al Hawaisah, Natih,

Fahud, and Daleel fields.

Sandstones of the Haushi Group (Gharif and

Al Khlata Formations—Khuff limestone seal)

form reservoirs in some fields. The Barik

sandstone member (Mabrouk Shale seal) and

Miqrat Formation (Al Bashair Shale seal) of

the Lower Cambrian to Lower Ordovician age

parts of the Haima Supergroup are primarily

deep gas reservoirs with reservoir potential

recently recognized in Precambrian carbonates

of the Buah Formation.

Similar to the Ghaba Salt Basin, most of the

fields of the Fahud Salt Basin are structurally

complex, salt-induced anticlines and domes

that have been broken up into several fault

blocks by crestal collapse features (Nederlof

and others, 1995). Specific common trap styles

are faulted closures, dip closures, and faulteddip closures (Lake, 1996).
Gulf of Oman

Ghaba Salt Basin

(2014)

Fahud Salt Basin

(2016)

Maradi Fault Zone

Rub' al Khali Basin

(2019)

Oman Mountains

(2017)

Gulf of Oman Basin

(2018)

55° E. 56° E. 57° E. 58° E. 59° E.

21°

23°

24°

22°

Saudi

Arabia

U.A.E.

Oil or gas

field centerpoint

(2016)

USGS Province

boundary

USGS province number USGS TPS number USGS assessment unit number

Assessment unit and Total Petroleum System (TPS) boundary

Pod of active

source rock boundary

Minimum Petroleum

System boundary

Country

boundary

EXPLANATION

Fahud-Natih

faults

Figure 14. Map showing Fahud Salt Basin Province (2016), North Oman Huqf -- Shu’aiba(!) Total Petroleum

System (201601), and Fahud-Huqf Combined Structural Assessment Unit (20160101). Oil and gas field

centerpoints (Petroconsultants, 1996) and boundaries for pod of active source rock and minimum petroleum

system are also shown. Scale = 1:2,750,000.

0 100 KM 

Central Oman Platform

20160101 (2015)

[201601] 20160101

North Oman Huqf - Shu'aiba TPS

[201601]

Foreland

Bulge

27
28

Source Rock Character

The geochemical characteristics of North

Oman Huqf-type oils are discussed in the

previous section on the North Oman Huqf/“Q”–

Haushi(!) TPS and are also summarized in

table 1. North Oman Huqf oils have ∂13C

compositions between about –33‰ and –35‰

and C27 sterane from 10 to 45 percent and can be

distinguished from other Huqf-sourced oils

(South Oman Huqf and “Q” types, see fig. 7).

Burial History, Generation, and Migration

Generation of hydrocarbons from Huqf

source rocks in North and Central Oman has

been discussed by Sykes and Abu Risheh (1989),

Visser (1991), Guit and others (1995),

Borgomano and others (1996), Droste (1997),

and Amthor and others (1998). Burial history

for the North Oman Huqf source rocks in the

Fahud Salt Basin Province is shown in figures

15 and 16.

Vitrinite reflectance equivalents of Huqf

source rocks within the the North Oman Huqf–

Shu’aiba(!) TPS ranges from about 2.0 to 4.0

percent (Sykes and Abu Risheh, 1989; and

Visser (1991). Burial history reconstructions by

Visser (1991) suggest that an early minor stage

of oil generation occurred in Middle and Lower

Huqf source rocks during the Early Silurian (fig.

15). Peak oil generation occurred during Late

Permian/Early Triassic (~250 Ma); gas

generation began during the Cretaceous (~110

Ma). Modelling by Amthor and others (1998)

suggests that gas expelled from Huqf source

rocks in the Fahud Salt Basin charged

structures across the Fahud Salt Basin and

reached the Makarem high during a period

ranging from 80 Ma to present day (fig. 16).

Fahud-Huqf Combined Structural Assessment

Unit (20160101)

One all-inclusive assessment unit,

designated as the Fahud-Huqf Combined

Structural Assessment Unit, has been assigned

to the North Oman Huqf–Shu’aiba(!) TPS of

the Fahud Salt Basin Province. The assessment

unit boundary is defined and described by the

Total Petroleum System boundary and includes

all reservoirs (Haima, Haushi, Shu’aiba,

Natih, among others) and styles of structural

traps (domes, anticlines, fault blocks, for

example) of the known inclusive fields that

produce hydrocarbons from Huqf sources in the

Fahud Salt Basin proper (fig. 14, table 2).

Thus, the assessment unit boundary outlines the

maximum geographic extent at which similar

undiscovered fields may exist in the North

Oman Huqf–Shu’aiba(!) TPS. The Fahud-Huqf

Combined Structural Assessment Unit shares a

common border to the south-southeast with the

Ghaba-Makarem Combined Structural

Assessment Unit (20140101) of the North Oman

Huqf/“Q”–Haushi(!) TPS (201401).

Similar to the Ghaba-Makarem Combined

Structural Assessment Unit (20140101),

expected exploration stategies and areas of

new-field discoveries in this assessment unit

are:

1. extension of Haima (Barik, Miqrat,

others) deep-gas/condensate play into the

Fahud Salt Basin with new-field discoveries

at depths >5,000 m; some additional targets

may also be discovered in reservoirs of the

underlying Buah Limestone, Huqf Supergroup.

2. growth of proven gas/condensate

reserves from Haima Supergroup in existing

fields (field growth).

3. extension of exploration into Fahud

Salt Basin for new oil fields and oil-field

growth within clastic reservoirs of the Haushi

Group (Al Khlata and Gharif).

4. new fields in Shu’aiba and Natih

limestones in fractured, low-relief, “pancake”

structures (all large structures have been

drilled) in western portion of assessment unit

and along western border of Oman, easternflank portion of the Rub’ al Khali Province

(2019).

5. growth of proven oil reserves in all

reservoirs of existing fields (field growth).

It is predicted in this study that a large

portion of the exploration efforts in Fahud Salt

Basin Province will focus on the Haima

Supergroup, and possibly Huqf Supergroup,

deep-gas/condensate play. Because much of

the gas resource is likely to be discovered in

existing fields, a large gas growth factor, the

Mid-Continent growth factor derived from the
VRE = 1.20

VRE = 2.40

VRE = 0.62

670 600 505 438 408 360 286 248 213 144 65 2

Huqf

oil

Huqf

gas

TD

Precambrian Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Tertiary

Fahud Salt Basin

6000

7000

4000

5000

3000

2000

1000

0

Depth (meters)

633

841

1391

2142

2518

3747

4011

5000

6000

6500

Ma bp

Generation -- Upper Huqf

Generation --Middle Huqf

HC expelled HC expelled

Cumulative

Cumulative

Upper Huqf

Upper Huqf

Middle Huqf

Middle Huqf

Figure 15. Burial-history diagram, vitrinite reflectance equivalent (VRE), and stages of hydrocarbon (HC) generation for Huqf

source rocks from unidentified well in vicinity of Yibal field, Fahud Salt Basin Province, Oman. Modified from Visser (1991).

29
610 570 510 476 439 409 386 363 323 290 256 208 178 148 97 65 35 5 Ma

6000

5000

2000

1000

3000

4000

0

Depth (meters)

Barik Sandstone Mbr

140 °C

120 °C

80 °C

100 °C Oil charge of early,

low-maturity

Huqf post-salt source

Huqf peak

oil generation

Huqf

gas charge

Jaleel Field Burial History

Biodegradation/porosity loss

pyrobitumen

Figure 16. Burial-history diagram for deep-gas reservoirs, Upper Cambrian-Lower Ordovician Barik Sandstone Member, showing temperatures

and stages of oil and gas charge from Huqf post-salt source rocks, Jaleel field, Fahud Salt Basin. Modified from Borgomano and others (1996).

30
31

U.S. Geological Survey 1995 National Oil and

Gas Assessment, was used in the assessment of

resources.

Size and number of new economic discoveries

may be limited by or dependent on the

following:

1. seismic resolution of low relief

structures and presence/absence of fractures or

leaching in Shu’aiba reservoirs with smaller

closure.

2. reduced porosity in deep Gharif targets

(Guit and others, 1995) in Fahud Salt Basin

which are likely to contain gas rather than oil.

3. thin, poor-quality sandstone reservoirs

and commonly the presence of pore-plugging

bitumen in Haima deep-gas reservoirs of the

Fahud Salt Basin.

4. poor-quality, high-nitrogen content of

gas along Oman Mountain front and overthrust.

Middle Cretaceous Natih(!) Total Petroleum

System (201602)

Although about 80 percent of the

hydrocarbons produced in Oman are thought to

be generated by source rocks of the

Infracambrian Huqf Supergroup (Sykes and Abu

Risheh, 1989), the middle Cretaceous Natih(!)

TPS is a smaller (about 20,000 km2 in

geographic extent) but highly efficient

petroleum system (Terken, 1998; in press). The

Natih(!) TPS is contained primarily (78 areal

percent) within the Fahud Salt Basin Province

with an estimated in-place resource volume of

some 9 BBOE (Terken, 1998; in press); this inplace volume is exclusive of hydrocarbons

generated by Huqf sources. About 2 billion

barrels of discovered recoverable reserves

attributed to the middle Cretaceous Natih(!)

TPS are concentrated in Fahud and Natih

fields.

The geographic extent of (1) the pod of

active source rock, (2) minimum petroleum

system, and (3) Total Petroleum System for the

Natih(!) TPS are shown in figure 17. The pod

of active source rock was determined from a

series of burial history reconstructions,

geothermal gradients, and thermal maturity

data (mostly vitrinite reflectance). The

minimum petroleum system boundary incorporates the pod of active source rock and all

known fields and oils shows from wells that

have produced “Natih-type” oil. The

maximum extent of the Middle Cretaceous

Natih(!) TPS is structurally bound to the north

by the Oman Mountains, to the south and

southwest by the Late Cretaceous to Tertiary

foreland bulge, and to the southeast by the

salt-structured core of the Ghaba Salt Basin

(figs. 2 and 17).

Source Rock Lithology and Geochemistry

The 400-m-thick carbonate sequence of the

Natih Formation is comprised of seven

lithologic subdivisions designated A through

G. Two organic-rich shaly intervals, the

Natih “B” and “E” units, that are easily

identified on well logs and of limited

geographic extent, have sourced the hydrocarbons of the Natih(!) TPS. In particular, the

500-m-thick Natih “B” unit is of excellent

source rock quality, having TOC contents as

high as 15 weight percent and averaging about

5 percent. These units contain predominantly

structureless Type I/II organic matter

(Grantham and others, 1988; van Buchem and

others, 1996; Terken, in press).

Natih oils have an API gravity of about 32°

and are distinctly different in geochemical

composition than other oils in Oman. Natih

oils have similar contributions from C27, C28,

and C29 steranes rather than a particular

dominance of one, which is characteristic of

Huqf and “Q” oils (table 1). Modeling by

Terken (1998; in press) indicates that only

minor gas has been generated from these oilprone, Natih source rocks.

Reservoirs, Seals, and Trap Styles

Porous (30–40 percent) carbonates of the

Natih A, C, D, and E intervals constitute the

reservoirs for oils generated by Natih source

rocks. Lithoclast and skeletal grainstone

aprons (shoals) and rudist packstone bioherms,

where fresh-water leaching has enhanced

porosities, compose many of the Natih

reservoirs (Harris and Frost, 1984). In contrast,

Natih field itself produces from heavily

fractured, low-permeability (0.5 to 10 mD),
0 100 KM

Gulf of Oman

Ghaba Salt Basin

(2014)

Fahud Salt Basin

(2016)

Maradi Fault Zone

Rub' al Khali Basin

(2019)

Oman Mountains

(2017)

Gulf of Oman Basin

(2018)

55° E. 56° E. 57° E. 58° E. 59° E.

21°

23°

24°

22°

Saudi

Arabia

U.A.E.

Oil or gas

field centerpoint

(2016)

USGS Province

boundary

USGS province number USGS TPS number USGS assessment unit number

Assessment unit and Total Petroleum System (TPS) boundary

Pod of active

source rock boundary

Minimum Petroleum

System boundary

Country

boundary

EXPLANATION

Figure 17. Map showing Fahud Salt Basin Province (2016), North Oman Middle Cretaceous Natih(!)

Total Petroleum System (201602), and Natih/Fiqa Structural-Stratigraphic Assessment Unit (20160201). 

Oil and gas field centerpoints (Petroconsultants, 1996) and boundaries for pod of active source rock

and minimum petroleum system also shown. Scale = 1:2,750,000.

Central Oman Platform

(2015)

20160201

[201602] 20160201

Natih TPS

[201602]

Foreland

Bulge

Fahud-Natih

faults

32
33

chalky limestones (Whyte, 1995; Terken, 1998).

Individual Natih reservoirs are sealed by the

intra-formational marls and shales; a thick

shale sequence of the overlying Fiqa Formation

(fig. 5) forms a major regional seal for the

Natih Formation. Natih oils are also found in

the Shu’aiba Formation in fault-dip structures

of Natih and Fahud fields. Natih oils in these

fields are explained by juxtaposition against

marginally mature Natih source rocks (Terken,

1998; in press). Future exploration may focus on

potential reservoirs within the overlying Fiqa

Formation where possible turbidite stratigraphic traps and truncation traps below lower

Fiqa shales have been recognized (Lake, 1996;

Terken, 1998).

Most traps are structural and related to

development of the foreland basin during the

Late Cretaceous/Tertiary. These structures

formed during two major stages of tectonics that

built the Oman Mountains. Obduction and

deformation during the first alpine event

produced normal and strike-slip faults, while

the second alpine event resulted in reactivation

and inversion of earlier faults near the thrust

front, most of which were enhanced by

halokinesis during these events (Loosveld and

others, 1996; Terken, 1998; in press).

Burial History, Generation, and Migration

The burial and thermal history of Natih

source rocks has been discussed by Harris and

Frost (1984), Terken (1998; in press) and, to a

lesser degree, by Visser (1991). Additional

data, information, and diagrams on thermal

maturity, as confidential reports contributed

from major oil companies, were also

incorporated into our study. Burial history of

the Natih Formation is illustrated in figures 18

and 19 and also summarized in the petroleum

system events chart of figure 10.

The Natih “kitchen” is defined where the

extent of the organic-rich facies is present in

the deepest parts of the foreland basin. The

pod of active source rock was originally

assigned using a series of burial-history

diagrams and approximated at the 2,000-mdepth structure contour of the Natih Formation;

however, subsequent modifications were made

after later reports and communications with

J.M.J. Terken (1998, Petroleum Development

Oman). Terken (1998, in press) described a

shallower extension of active source rock of

lesser thermal maturity to the east of the

Fahud and Natih faults and along the Maradi

fault zone (fig. 1), which is an area of high (up

to 28 °C/km) geothermal gradient.

The thickness of the massive Fiqa shale seal

and modest folding and thrusting of the Oman

Mountains forced lateral migration of Natih

oils. Modeling of the Natih by Terken (1998, in

press) shows that migration was initially

towards the foreland bulge and Ghaba Salt

Basin but was interrupted by the formation of

the Fahud fault during early development of

the foreland basin. The fault thus created a

shadow zone preventing migration of Natih

oils to reach the foreland bulge and into Yibal

and Al Huwaisah fields (fig. 2). The Natih

structure, however, formed during the second

alpine event and subsequently deflected the

hydrocarbon charge from the Fahud field. The

high integrity of the massive Fiqa top seal has

allowed hydrocarbons to be retained in these

shallow, large-throw fault-dip structures.

Only minor gas has been generated from this

highly oil prone source with some gas

migration towards the Lekhwair area and

Maradi fault zone (Terken, 1998, in press).

Natih-Fiqa Structural/Stratigraphic

Assessment Unit (20160201)

One all-inclusive assessment unit,

designated as the Natih-Fiqa Structural/

Stratigraphic Assessment Unit, has been

assigned to the middle Cretaceous Natih(!)

TPS of the Fahud Salt Basin Province. The

assessment unit boundary is defined and

described by the Total Petroleum System

boundary and includes all known fields and

wells that produce oils, or have oil shows, that

have been identified geochemically as Natih

sourced (fig. 17, table 2). Thus, the Natih-Fiqa

Structural/Stratigraphic Assessment Unit

boundary outlines the maximum geographic

extent at which similar undiscovered fields

may exist in the middle Cretaceous Natih(!)

TPS and is independent of the North Oman

Huqf–Shu’aiba(!) TPS (201601).
Shu'aiba

Nahr Umr

Natih

Fiqa

Hadhramaut

Fars

1000

2000

3000

4000

125 100 75 50 25

Million Years

MESOZOIC

Cretaceous Paleogene

CENOZOIC

Neogene

Kharaib

40 °C

60 °C

80 °C

100 °C

120 °C

140 °C

VRE = 0.8

VRE = 0.62

VRE = 1.0 

VRE = 0.55

UNIT

GEOLOGIC

TIME

SCALE

DEPTH (METERS)

0

Figure 18. Burial-history diagram, burial temperatures, and vitrinite reflectance equivalent (VRE) for Natih Formation source

rocks in Late Cretaceous/Tertiary foredeep, Fahud Salt Basin, north Oman. Modified from Terken (1998; in press).

34
120 100 80 60 40 20 0

Early Cret. CENOZOIC

Bar. Apt. Albian Cen. Tur. C. S. Cam. M. Pal. Eocene Olig. Miocene

0 ft/m

2,000 ft/600 m

Neogene

Oliogocene

Eocene

Paleocene

Fiqa Shale

Natih ls

Nahr Umr

Thamama

Group

10,000 ft/3,000 m

8,000 ft/2,400 m

6,000 ft/1,800 m

4,000 ft/1,200 m

Middle Cret. Late Cret.

Figure 19. Burial-history diagram and major diagenetic events of Wasia Group near Anaima-1 well,

Fahud Salt Basin, north Oman. Modified from Harris and Frost (1984).

Maturation of

Jurassic - L. Cretaceous

Sea

Level

Fresh-water leaching

Formation

of

Omani

Foredeep

Stylolitization

35
36

At present, only eight fields (table 2) are

allocated to the middle Cretaceous Natih(!)

TPS and Natih-Fiqa Structural/Stratigraphic

Assessment Unit with most of the recoverable

oil in the two giant fields, Fahud and Natih.

Subsequently, only smaller, marginally economic discoveries were made in fields along the

Maradi fault zone. Shows of Natih oils have

also been identified in at least seven other

fields or wells (Terken, in press).

Exploration and new-field discoveries in

this assessment unit are estimated using the

data and interpretations of Lake (1996),

Loosveld and others (1996), and Terken (in

press). Future discoveries will be from more

subtle trap styles, including:

1. structural traps related to foreland

basin development that include faulteddip/truncation closures on the northern flank of

the foreland bulge, and traps above salt domes

with late Tertiary movement.

2. stratigraphic traps within Natih

carbonate buildups.

3. stratigraphic traps in unproven

turbidites or other marine clastics in Fiqa

Formation, foreland basin area.

Size and number of new economic discoveries

may be limited due to:

1. presence/absence of the massive top

Fiqa shale seal.

2. limited areal extent and maturity of

Natih organic-rich facies in Late Cretaceous

foredeep.

3. charge shielding by major fault zones of

laterally migrated Natih oils into distal

structures.

Summary

Three Total Petroleum Systems and their

associated assessment units have been interpreted in this study for the Ghaba and Fahud

Salt Basin Provinces of north-central Oman.

The boundary for each Total Petroleum System

also defines a boundary that corresponds to an

assessment unit and incorporates all trap styles

and reservoirs of the produced hydrocarbons.

In both the Ghaba and Fahud Salt Basins,

hydrocarbons were generated from several

deeply buried source-rock units of the

Infracambrian Huqf Supergroup. Although one

general “North Oman Huqf” type oil is

dominant in the Fahud Salt Basin, oils in the

Ghaba Salt Basin can be linked to at least two

distinct Huqf source-rock units, commonly found

as admixtures, the general North Oman Huqftype oil source and a more dominant

“questionable unidentified-source” or “Q”-type

Huqf oil source.

Hydrocarbons generated from Huqf sources

are produced from a variety of reservoir types

and ages in both the Ghaba and Fahud Salt

Basin Provinces. Clastic reservoirs of the

Gharif and Al Khlata Formations, Haushi

Group (middle Carboniferous to Late Permian),

dominate oil production in the Ghaba Salt

Basin Province. In contrast, Cretaceous

carbonates of the Shu’aiba and Natih

limestones account for most of the production in

the Fahud Salt Basin. Deep gas is produced

mainly from Middle Cambrian to Lower

Ordovician clastic reservoirs of the Haima

Supergroup with future potential in deeper

Huqf reservoirs. Traps are mainly structural in

origin and were formed by one or more

mechanisms during periodic halokinesis of the

thick Cambrian Ara Salt beginning with the

deposition of the thick Haima clastics and

from consequent folding and faulting from basin

loading, rifting, and tectonics forming the

Oman Mountains and associated foreland-basin

system during the Late Cretaceous and early

Tertiary. Most of the future new-field

discoveries will likely target low-relief

structures, as many of the large structures have

been drilled, and will target deep gas in the

Haima and Huqf Supergroups.

The middle Cretaceous Natih Total

Petroleum System is a small efficient system of

the Fahud Salt Basin Province producing

primarily from Natih reservoirs along faultdip structures. Most traps are structural and

are related to development of the foreland

basin and formation of the Oman Mountains.

Future targets will be less obvious than those of

Fahud and Natih fields, and frontier

exploration will include stratigraphic traps

within Natih buildups and in unproven

turbidite and other marine clastics of the Fiqa

Formation.
37

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40

Table 1. Common characteristics of source rocks and oils of Ghaba and Fahud Salt Basin Provinces,

north-central Oman.

[TOC, total organic carbon in weight percent: OMt, organic-matter type; %S, percent sulfur in oil; Xc, presence/absence of X-branched compounds; N.D., no data]

Source rock/ OMt TOC ∂13C (‰) Sterane % API %S X-c

oil type (%) (C27, C28, C29)

North Oman Huqf I/II 3 –33 to –35 20,20,60 25°–45° 1.0–2.0 yes

Huqf “Q” I/II N.D. –30 to –31 63,22,15 40°–50° 0.1–0.4 yes

Natih I/II 5 –27 34,38,28 31°–32° 1.5 no
41

Table 2. Allocation of known oil and gas fields to Total Petroleum Systems and Assessment Units of the

Ghaba and Fahud Salt Basin Provinces, Oman.

[Gas fields indicated in bold type]

201401 North Oman Huqf/“Q’’–Haushi(!) Total Petroleum System

20140101 Ghaba-Makarem Combined Structural Assessment Unit

Abber 1 Burhaan West Jerian 1 Nafoorah 1 Saih Rawl

Alam Burhaan North Jebel Madar Nahda 1 Safwan 2

Al Ghubar Burhaan South Katheer 1 Qarat Al Milh Sayyala

Anbar 1 Farha South Mabrouk Qarat Al Milh 1 Shaheer 1

Anzuaz Fayyadh Mafraq 3 Qarn Alam Suwaihat

Asfoor 2 Ghaba East Mahfil 1 Rajaa Tawf Dahm 1

Bahaa 1 Ghufos Mahjoub 2S1 Ramlat Rawl Wafra

Bahja Habur Malih 1 Rasafah 1 Zareef

Baqlah 1 Hadiyah 1 Maqtaa Risalah 1 Zualiyah

Barakat 2 Hasirah Mazeed 1 Sadad

Barik Hawqa 1 Misfar 1 Saih Nihayda

Barik North Hijab 1 Mussalim Saih Nihayda SE

201601 North Oman Huqf–Shu’aiba(!) Total Petroleum System

20160101 Fahud-Huqf Combined Structural Assessment Unit

Al Aroos Daleel Jaleel 1st Maqhoul South 1 Thumayd 1

Al Barakah Fahud West (gas) Jebel Aswad 1 Maradi Wadi Latham

Al Barakah NE 1 Fushaigah 1 Khatmah 1 Mazrouq 1 Wadi Rafash 1X

Al Bashair 1 Haban 1 Khulad 1 Mezoon 1 Wadi Umayri 1

Al Husain 1 Habiba 1 Lobnah 1 Nadir 1 Yibal

Al Huwaisah Hafar 1 Madiq Natih West (gas)

Al Sahwa 1 Hareer 1 Makarem 1 (ST2) Salmah

Bushra 1 Ibtikar 1ST Malih 1 Salmah South

201602 Middle Cretaceous Natih(!) Total Petroleum System

20160201 Natih-Fiqa Structural/Stratigraphic Assessment Unit

Natih Fahud West (oil) Shipkah Maradi (Ladah) 2

Natih West (oil) Fahud Suqtan 1 Qalah 1